A typical gas turbine engine, such as an axial flow gas turbine engine, includes a compressor section, a turbine section, and a combustor section. This combustor section is located between the compressor section and the turbine section, and produces high temperature gas by burning a mixture of fuel and compressed air. The combustor section is typically annular and has radially inner and outer walls. A nozzle structure, which is typically annular in shape, has an inlet which mates, and is radially coextensive, with the combustor section's inner and outer walls at the outlet side of the combustor section of the gas turbine engine.
A conventional nozzle structure is positioned in its desired location within a gas turbine engine by clamping the nozzle structure between axial adjacent faces of a supporting structure. This conventional nozzle structure normally includes a radially outer shroud, a radially inner shroud, and a plurality of airfoil vanes. Each of the airfoil vanes extends radially between the outer and inner shrouds, and has a first end fixed to the outer shroud and a second end fixed to the inner shroud.
These airfoil vanes of the nozzle structure are arranged to optimally direct the high temperature gas produced by the combustor section toward the blades of a turbine wheel assembly in the gas turbine engine's turbine section. Accordingly, the high temperature gas accelerates through the nozzle structure in order to thermodynamically and aerodynamically engage the blades mounted on the turbine wheel of the turbine section.
Because of efforts to maintain the combustor section's walls below a reasonable temperature, the temperature profile of the high temperature gas exiting the combustor section is normally highest at its midpoint, which is radially intermediate the inner and outer walls of the combustor section. Moreover, the temperature of the gas produced by a combustor section of such a gas turbine engine is constantly being increased to improve the efficiency, and thereby the fuel economy, of the gas turbine engine. Increasing the temperature of the gas produced by a combustor section of a gas turbine engine results in an increased temperature at the inlet side of the gas turbine engine's nozzle structure.
Because of such high outlet temperatures of the gas exiting from the combustor section, and because the temperature profile of the high temperature gas exiting the combustor section directs the hottest portion of this gas onto the airfoil vanes of the nozzle structure, ceramic is being used for the material of nozzle structures, particularly for the airfoil vanes, since ceramic functions very well in high temperature environments. Ceramic nozzle structures are, however, typically mounted on metal supporting structures which commonly constitute the majority of the structural members in a gas turbine engine. Differential thermal expansion between a ceramic nozzle structure and its metal supporting structure results in high, potentially damaging thermal stresses, particularly where the ceramic airfoil vanes are clamped between outer and inner shrouds.
The assignee of the present invention has recently developed a cantilevered ceramic nozzle structure employing a radially outer shroud having airfoil vanes connected at one end thereto, and protruding radially inwardly therefrom. That is, first ends of the airfoil vanes are connected to the outer shroud, and second ends of the airfoil vanes extend radially toward the inner shroud. However, the radially inner shroud is radially spaced apart from the second ends of the airfoil vanes. Therefore, the second ends of the airfoil vanes may be referred to as free ends. Because the second ends of the airfoil vanes are free ends, thermal stress on the airfoil vanes is substantially reduced.
In such a cantilevered ceramic nozzle structure, the outer shroud is supported to a nozzle support ring by pins which extend through pin receiving holes in the outer shroud and corresponding pin receiving holes in the nozzle support ring. In this arrangement, however, great care must be taken to ensure that each of the pin receiving holes of the outer shroud and its corresponding pin receiving hole of the nozzle support ring are nearly perfectly aligned in order to avoid stressing the outer shroud and/or the nozzle support ring. The present invention provides a cantilevered nozzle structure which avoids the use of pins for supporting airfoil vanes to a nozzle support ring.